Fertilization is the process by which sex cells unite to form a zygote, or fertilized egg. Fertilization results in a new cell with a unique mix of genes. Sex cells (sperm or eggs) contain half the number of chromosomes normally found in the cells of an organism. Fertilization results in a complete set of chromosomes.

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Sea urchin embryo cell division

The first 90 minutes of sea urchin (Lytechinus pictus) embryo development. Nuclei are visible as “thumbprints” in each cell. Microtubules are faintly visible just prior to and during mitosis.

Elapsed time about 2 hours.

Sea urchin eggs were fertilized, then immediately mounted in seawater between a slide and coverslip (using a silicon spacer). Images were taken at room temperature on a compound inverted microscope using a 40x DIC objective and digital camera.